Cognitive Radio System, The Nc-ofdm Channel Estimation Technology Research,

With the growth of broadband wireless communication and the diversification of wireless services, Cognitive Radio (CR) attracts more and more attention as it is an effective solution for the contradiction between spectrum scarcity and waste. As Non-Contiguous Orthognal Frequency Division Multiplexing (NC-OFDM) can flexibly exploit usable frequency band, it has been proposed as a promising candidate for high data rates transmition in CR context. To fully make use of the advatange of NC-OFDM, channel estimation should be perofmed to obtain the channel estate information. This thesis focuses on the reseach on channel estimation technologies for NC-OFDM systems in CR context.In Chapter 1, the development and current status of CR are overveiwed. For the research on channel estimation is based on the overwhelming understanding of wireless channel, and any proposed novel channel estimation methodology need to be verified based a propagation channel model, the wireless channel theory is discussed at firest. Then, the Space Channel Model Extention (SCME) channel model is investigated in Chapter 2. The simulating and evaluation of SCME channel model are provided.The current channel estimation method for single-input single-output (SISO) systesm depends greatly on Winner filter, which has high complexity. Morever, in order to ensure the accuracy of Winner filter, a complicated pilot insertion method has to be adopted. In Chapter 3 of this thesis, a new channel estimation method is proposed to address these issues. In the proposed channel estimation, the Least Square (LS) channel estimation is applied to obtain the channel fading coefficients associated with the subcarriers over which the pilot symbols are transmitted. Furthermore, the time domain channel pulse response is computed based on the basic principle of Fourier transform. Then, the inverse discrete Fourier transforms (IDFT) based channel estimation noise reduction is used to improve the channel estimation accuracy. For slow time-varying channel, pilot-block average based channel estimation noise reduction is also used to further improve the channel estimation performance. Simulation results validate our proposed channel estimation method.In Chapter 4, the channel estimation for multiple-input multiple-output (MIMO) NC-OFDM systems is investigated. The system structure and characterristics of MIMO NC-OFDM are introduced at first. Based the analysis of MIMO NC-FODM, the pilot-pattern is discussed, which is very important for MIMO systems. It is proved that orthogonal pilot matrix could achieve minimum Mean Square Error (MSE) estimation, and the method to compose such a matrix is given. The channel estimation in SISO NC-OFDM system was extended to fit MIMO system, in which different wireless channels were processed separately and the problems in MIMO system were transform into equivalent SISO ones. The total implementation complexity is also analyzed. At last the proposed channel estimation method is evaluated through computer simulation and the corresponding results are analysed.